Highly compact EPROM and flash EEPROM devices

ABSTRACT

Structures, methods of manufacturing and methods of use of electrically programmable read only memories (EPROM) and flash electrically erasable and programmable read only memories (EEPROM) include split channel and other cell configurations. An arrangement of elements and cooperative processes of manufacture provide self-alignment of the elements. An intelligent programming technique allows each memory cell to store more than the usual one bit of information. An intelligent erase algorithm prolongs the useful life of the memory cells. Use of these various features provides a memory having a very high storage density and a long life, making it particularly useful as a solid state memory in place of magnetic disk storage devices in computer systems.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of copending application Ser. No. 08/154,162, filedNov. 17, 1993, which is a division of application Ser. No. 07/777,673,filed Oct. 15, 1991, now U.S. Pat. No. 5,268,319, which is a division ofapplication Ser. No. 071381,139, filed Jul. 17, 1989, now U.S. PatentNo. 5,198,380, and which in turn is a division of original applicationSer. No. 07/204,175, filed Jun. 8, 1988, now U.S. Pat. No. 5,095,344.The foregoing patents are also incorporated herein by this reference.

BACKGROUND OF THE INVENTION

This invention relates generally to semi-conductor electricallyprogrammable read only memories (Eprom) and electrically erasableprogrammable read only memories (EEprom), and specifically tosemiconductor structures of such memories, processes of making them, andtechniques for using them.

An electrically programmable read only memory (Eprom) utilizes afloating (unconnected) conductive gate, in a field effect transistorstructure, positioned over but insulated from a channel region in asemi-conductor substrate, between source and drain regions. A controlgate is then provided over the floating gate, but also insulatedtherefrom. The threshold voltage characteristic of the transistor iscontrolled by the amount of charge that is retained on the floatinggate. That is, the minimum amount of voltage (threshold) that must beapplied to the control gate before the transistor is turned "on" topermit conduction between its source and drain regions is controlled bythe level of charge on the floating gate. A transistor is programmed toone of two states by accelerating electrons from the substrate channelregion, through a thin gate dielectric and onto the floating gate.

The memory cell transistor's state is read by placing an operatingvoltage across its source and drain and on its control gate, and thendetecting the level of current flowing between the source and drain asto whether the device is programmed to be "on" or "off" at the controlgate voltage selected. A specific, single cell in a two-dimensionalarray of Eprom cells is addressed for reading by application of asource-drain voltage to source and drain lines in a column containingthe cell being addressed, and- application of a control gate voltage tothe control gates in a row containing the cell being addressed.

This type of Eprom transistor is usually implemented in one of two basicconfigurations. One is where the floating gate extends substantiallyentirely over the transistor's channel region between its source anddrain. Another type, preferred in many applications, is where thefloating gate extends from the drain region only part of the way acrossthe channel. The control gate then extends completely across thechannel, over the floating gate and then across the remaining portion ofthe channel not occupied by the floating gate. The control gate isseparated from that remaining channel portion by a thin gate oxide. Thissecond type is termed a "split-channel" Eprom transistor. This resultsin a transistor structure that operates as two transistors in series,one having a varying threshold in response to the charge level on thefloating gate, and another that is unaffected by the floating gatecharge but rather which operates in response to the voltage on thecontrol gate as in any normal field effect transistor.

Early Eprom devices were erasable by exposure to ultraviolet light. Morerecently, the transistor cells have been made to be electricallyerasable, and thus termed electrically erasable and programmable readonly memory (EEprom). One way in which the cell is erased electricallyis by transfer of charge from the floating gate to the transistor drainthrough a very thin tunnel dielectric. This is accomplished byapplication of appropriate voltages to the transistor's source, drainand control gate. Other EEprom memory cells are provided with aseparate, third gate for accomplishing the erasing. An erase gate passesthrough each memory cell transistor closely adjacent to a surface of thefloating gate but insulated therefrom by a thin tunnel dielectric.Charge is then removed from the floating gate of a cell to the erasegate, when appropriate voltages are applied to all the transistorelements. An array of EEprom cells are generally referred to as a FlashEEprom array because an entire array of cells, or significant group ofcells, is erased simultaneously (i.e., in a flash).

EEprom's have been found to have a limited effective life. The number ofcycles of programming and erasing that such a device can endure beforebecoming degraded is finite. After a number of such cycles in excess of10,000, depending upon its specific structure, its programmability canbe reduced. Often, by the time the device has been put through such acycle for over 100,000 times, it can no longer be programmed or erasedproperly. This is believed to be the result of electrons being trappedin the dielectric each time charge is transferred to or away from thefloating gate by programming or erasing, respectively.

It is the primary object of the present invention to provide Eprom andEEprom cell and array structures and processes for making them thatresult in cells of reduced size so their density on a semiconductor chipcan be increased. It is also an object of the invention that thestructures be highly manufacturable, reliable, scalable, repeatable andproducible with a very high yield.

It is yet another object of the present-invention to provide EEpromsemiconductor chips that are useful for solid state memory to replacemagnetic disk storage devices.

Another object of the present invention is to provide a technique forincreasing the amount of information that can be stored in a given sizeEprom or EEprom array.

Further, it is an object of the present invention to provide a techniquefor increasing the number of program/read cycles that an EEprom canendure.

SUMMARY OF THE INVENTION

These and additional objects are accomplished by the various aspects ofthe present invention, either alone or in combination, the primaryaspects being briefly summarized as below:

1. The problems associated with prior art split channel Eprom and splitchannel Flash EEprom devices are overcome by providing a split channelmemory cell constructed in one of the following ways:

(A) In one embodiment, one edge of the floating gate is self aligned toand overlaps the edge of the drain diffusion and the second edge of thefloating gate is self aligned to but is spaced apart from the edge ofthe source diffusion. A sidewall spacer formed along the second edge ofthe floating gate facing the source side is used to define the degree ofspacing between the two edges. Self alignment of both source and drainto the edges of the floating gate results in a split channel Epromdevice having accurate control of the three most critical deviceparameters: Channel segment lengths L1 and L2 controllable by floatinggate and control gate, respectively, and the extent of overlap betweenthe floating gate and the drain diffusion. All three parameters areinsensitive to mask misalignment and can be made reproducibly very smallin scaled-down devices.

(B) In a second embodiment of the split channel Eprom a heavily dopedportion of the channel adjacent to the drain diffusion is formed by anovel,, well-controlled technique. The length Lp and dopingconcentration of this channel portion become the dominant parameters forprogramming and reading, thereby permitting the formation of a splitchannel structure which is relatively insensitive to misalignmentsbetween the floating gate and the source/drain regions.

2. A separate erase gate is provided to transform a Eprom device into aFlash EEprom device. The area of overlap between the floating gate andthe erase gate is insensitive to mask misalignment and can therefore bemade reproducibly very small.

3 . In some embodiments of this invention, the erase gate is also usedas a field plate to provide very compact electric isolation betweenadjacent cells in a memory array.

4. A new erase mechanism is provided, which employs tailoring of theedges of a very thin floating gate so as to enhance their effectivenessas electron injectors.

5. A novel intelligent programming and sensing technique is providedwhich permits the practical implementation of multiple state storagewherein each Eprom or flash EEprom cell stores more than one bit percell.

6. A novel intelligent erase algorithm is provided which results in asignificant reduction in the electrical stress experienced by the erasetunnel dielectric and results in much higher endurance to program/erasecycling.

The combination of various of these features results in new splitchannel Eprom or split channel Flash EEprom devices which are highlymanufacturable, highly scalable, and offering greater storage density aswell as greater reliability than any prior art Eprom or Flash EEpromdevices. Memories that utilize the various aspects of this invention areespecially useful in computer systems to replace existing magneticstorage media (hard disks and floppy disks), primarily because of thevery high density of information that may be stored in them.

Additional objects, features and advantages of the present inventionwill be understood from the following description of its preferredembodiments, which description should be taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of the split channel Flash EEprom Samachisaprior art cell which erases by tunneling of electrons from the floatinggate to the drain diffusion.

FIG. 2a is a cross section of the Flash EEprom Kynett prior art cellwhich erases by tunneling of electrons from the floating gate to thesource diffusion.

FIG. 2b is a cross section of the Flash EEprom Kupec prior art cell withtriple polysilicon.

FIG. 2c is a schematic of the Kupec cell during erase.

FIG. 3a is a topological view of the triple polysilicon split channelFlash EEprom prior art Masuoka cell which erases by tunneling ofelectrons from the floating gate to an erase gate.

FIG. 3b is a schematic view of the Masuoka prior art cell of FIG. 3a.

FIG. 3c is a view of the Masuoka prior art cell of FIG. 3a along crosssection AA.

FIG. 3d is a cross section view of the split channel Eprom Harari priorart cell.

FIG. 4a is a cross section view of the split channel Eprom Eitan priorart cell having a drain diffusion self aligned to one edge of thefloating gate.

FIG. 4b is a cross section view of the prior art Eitan cell of FIG. 4aduring the process step used in the formation of the self aligned draindiffusion.

FIG. 4c is a cross section view of the split channel Eprom Mizutaniprior cell with sidewall spacer forming the floating gate.

FIG. 4d is a cross section view of the split channel Eprom Wu prior artcell with sidewall spacer forming one of two floating gates.

FIG. 4e is a cross section view of a stacked gate Eprom Tanaka prior artcell with heavily doped channel adjacent to the drain junction.

FIG. 5a is across section of a split channel Eprom cell in accordancewith this invention.

FIGS. 5b through 5f are cross sections of the cell of FIG. 5a duringvarious stages in the manufacturing process.

FIG. 6a is a top view of a 2×2 array of Flash EEprom cells formed in atriple layer structure in accordance with one embodiment of thisinvention.

FIG. 6b is a view along cross section AA of the structure of FIG. 6a.

FIG. 7a is a top view of a 2×2 array of Flash EEprom cells formed in atriple layer structure in accordance with a second embodiment of thisinvention wherein the erase gates also provide field plate isolation.

FIG. 7b is a view along cross section AA of the structure of FIG. 7a.

FIG. 7c is a view along cross section CC of the structure of FIG. 7a.

FIG. 8a is a top view of a 2×2 array of Flash EEprom cells formed in atriple layer structure in accordance with a third embodiment of thisinvention wherein the tunnel erase dielectric is confined to thevertical surfaces at the two edges of the floating gate.

FIG. 8b is a view along cross section AA of the structure of FIG. 8a.

FIG. 9a is a top view of a 2'2 array of Flash EEprom cells formed in atriple layer structure in accordance with a fourth embodiment of thisinvention wherein the erase gate is sandwiched in between the floatinggate and the control gate.

FIG. 9b is a view along cross section AA of the structure of FIG. 9a.

FIG. 9c is a view along cross section DD of the structure of FIG. 9a.

FIG. 10 is a schematic representation of the coupling capacitancesassociated with the floatings gate of the Flash EEprom cell of theinvention.

FIG. 11a is a schematic representation of the composite transistorforming a split channel Eprom device.

FIG. 11b shows the programming and erase characteristics of a splitchannel Flash EEprom device.

FIG. 11c shows the four conduction states of a split channel FlashEEprom device in accordance with this invention.

FIG. 11d shows the program/erase cycling endurance characteristics ofprior art Flash EEprom devices.

FIG. 11e shows a circuit schematic and programming/read voltage pulsesrequired to implement multistate storage.

FIG. 12 outlines the key steps in the new algorithm used to erase with aminimum stress.

FIG. 13 shows the program/erase cycling endurance characteristics of thesplit channel Flash EEprom device of this invention using intelligentalgorithms for multistate programming and for reduced stress duringerasing.

FIGS. 14a, 14b and 14c are cross sections of another embodiment of thisinvention during critical steps in the manufacturing flow.

FIGS. 15a and 15b are schematic representations of two memory arrays forthe Flash EEprom embodiments of this invention.

FIGS. 16a and 16b are cross sectional views of Flash EEprom transistors,illustrating the erase mechanism by asperity injection (16a) and sharptip injection (16b).

FIGS. 16c and 16d are cross sectional views of parts of Flash EEpromtransistors illustrating the formation of sharp-tipped edges of thefloating gate by directional etching to facilitate high field electronicinjection.

FIG. 17a contains Table I which shows voltage conditions for alloperational modes for the array of FIG. 15a.

FIG. 17b contains Table II which shows example voltage conditions forall operational modes for the virtual ground array of FIG. 15b.

DETAILED DESCRIPTION OF THE PRIOR ART

There are two distinctly different approaches in the prior art of FlashEEproms. A triple polysilicon device was described by J. Kupec et al. in1980 IEDM Technical Digest, p. 602 in an article entitled "Triple LevelPolysilicon EEprom with Single Transistor per Bit". An improvement tothe Kupec device was proposed by F. Masuoka and H. Iizuka in U.S. Pat.No. 4,531,203, issued Jul. 23, 1985. Variations on the same cell aredescribed by C. K. Kuo and S. C. Tsaur in U.S. Pat. No. 4,561,004 issuedDec. 24, 1985, and by F. Masuoka et al. in an article titled "A 256 KFlash EEprom Using Triple Polysilicon Technology", Digest of TechnicalPapers, IEEE International Solid-State Circuits Conference, February1985, p. 168.

The second approach is a double polysilicon cell described by G.Samachisa et al., in an article titled "A 128 K Flash EEprom UsingDouble Polysilicon Technology", IEEE Journal of Solid State Circuits,October 1987, Vol. SC-22, No. 5, p. 676. Variations on this second cellare also described by H. Kume et al. in an article titled "A Flash-EraseEEprom Cell with an Asymmetric Source and Drain Structure", TechnicalDigest of the IEEE International Electron Devices Meeting, December1987, p. 560, and by V. N. Kynett et al. in an article titled "AnIn-System Reprogrammable 256 K CMOS Flash Memory", Digest of TechnicalPapers, IEEE International Solid-State Circuits Conference, February1988, p. 132. A cross-section of the Samachisa cell is shown in FIG. 1.Transistor 100 is an NMOS transistor with source 101, drain 102,substrate 103, floating gate 104 and control gate 109. The transistorhas a split channel consisting of a section 112 (L1) whose conductivityis controlled by floating gate 104, in series with a section 120 (L2)whose conductivity is controlled by control gate 109. Programming takesplace as in other Eprom cells by injection of hot electrons 107 from thechannel at the pinchoff region 119 near the drain junction. Injectedelectrons are trapped on floating gate 104 and raise the conductionthreshold voltage of channel region 112 and therefore of transistor 100.To erase transistor 100 the oxide in region 112 separating between thefloating gate 104 and drain diffusion 102 and channel 112 is thinned tobetween 15 and 20 nanometers, to allow electronic tunneling of trappedelectrons 108 from the floating gate to the drain. In the Samachisa cellthe appropriate voltages applied to achieve programming are V_(CG) =12V, V_(D) =9 V, V_(BB) =0 V, V_(S) =0 V, and to achieve erase are V_(CG)=0 V, V_(D) =19 V, V_(BB) =0 V, V_(S) =floating. Samachisa points outthat the electrical erase is not self-limiting. It is possible toovererase the cell, leaving the floating gate positively charged, thusturning the channel portion L1 into a depletion mode transistor. Theseries enhancement transistor L2 is needed therefore to preventtransistor leakage in the overerase condition.

The Samachisa cell suffers from certain disadvantages. These are:

(a) It is difficult to prevent avalanche junction breakdown or highjunction leakage current at the drain junction 102 during the time thevery high erase voltage is applied to the drain;

(b) It is difficult to grow with high yields the thin oxide layer 112used for tunnel erase;

(c) Because of the presence of thin oxide layer between the floatinggate and the drain diffusion, it is difficult to prevent accidentaltunneling of electrons from the floating gate to the drain in what isknown as the "program disturb" condition. Under this condition anunselected cell in a memory array sharing the same drain (bit line) as aprogrammed cell may have a drain voltage of approximately 10 volts and acontrol gate voltage of 0 volts. Although this represents a much weakerelectric field than that experienced during tunnel erase (when the drainis at approximately 19 volts), it nevertheless cart over a prolongedperiod of time alter by slow tunneling the charge stored on the floatinggate.

The Kynett and Kume cells (FIG. 2a) are similar to the Samachisa cellexcept for the elimination of the series enhancement transistor 120, andthe performing of tunnel erase 208 over the source diffusion 201 ratherthan over the drain diffusion 202. Typically the Kynett cell uses duringprogramming voltages V_(CG) =12 V, V_(D) =8 V, V_(S) =0 V, V_(BB) =0 V,and during erase voltages V_(S) =12 V, V_(BB) =0 V, V_(CG) =0 V, V_(D)=Floating. Kynett achieves a lower erase voltage than Samachisa bythinning tunnel dielectric 212 to 10 nanometers or less, so that eventhough the voltage applied to the source diffusion during erase isreduced, the electric field across tunnel dielectric 212 remains as highas in the case of the Samachisa cell.

The Kynett cell can be contrasted with the Samachisa cell:

(a) Kynett is less susceptible to avalanche breakdown of sourcediffusion 201 during erase because the voltage is reduced from 19 voltsto 12 volts.

(b) Kynett's cell is more susceptible to low yields due to pinholes inthe thin dielectric layer 212 because its thickness is reduced fromapproximately 20 nanometers to approximately 10 nanometers.

(c) Because Kynett uses a lower voltage for erase but essentially thesame drain voltage for programming Kynett is far more susceptible toaccidental "program disturb" due to partial tunnel erase (duringprogramming) occuring from floating gate 204 to drain 202.

(d) Kynett's cell is highly susceptible to an overerase conditionbecause it does not have the series enhancement channel portion 120 ofSamachisa's cell. To prevent overerase Kynett et al. deploy a specialerase algorithm. This algorithm applies a short erase pulse to an arrayof cells, then measures the threshold voltage of all cells to ensurethat no cell has been overerased into depletion. It then applies asecond erase pulse and repeats the reading of all cells in the array.This cycle is stopped as soon as the last cell in the array has beenerased to a reference enhancement voltage threshold level. The problemwith this approach is that the first cell to hive been adequately erasedcontinues to receive erase pulses until the last cell has beenadequately erased, and may therefore be susceptible to overerase into adepletion threshold state.

Kupec's cell employs essentially the Kynett cell without a thin tunneldielectric over the source, channel, or drain, and with a thirdpolysilicon plate covering the entire transistor and acting as an eraseplate. A cross sectional view of the Kupec device is shown in FIG. 2b.Transistor 200b consists of a stacked floating gate 204b and controlgate 209b with source 201b and drain 202b self aligned to the edges ofthe floating gate. Gate dielectric 212 is relatively thick and does notpermit tunnel erase from floating gate to source or drain. An eraseplate 230b overlies the control gate and covers the sidewalls of boththe control gate and the floating gate. Erase takes place by tunnelingacross the relatively thick oxide 231b between the edges of floatinggate 204b and erase plate 230b. Kupec attempts to overcome the overerasecondition by connecting the erase plate during high voltage erase todrain 202b and through a high impedance resistor R (FIG. 2c) to theerase supply voltage V_(ERASE). As soon as the cell is erased intodepletion the drain to source transistor conduction current drops mostof the erase voltage across the resistor, reducing the voltage on theerase plate 230b to below the tunneling voltage. This approach isextremely difficult to implement in a block erase of a large arraybecause different transistors begin conduction at different times.

Masuoka's approach to Flash EEprom overcomes most of the disadvantagesof the Samachisa, Kynett and Kupec cells. FIG. 3a provides a top view ofthe Masuoka prior art cell, FIG. 3b shows the schematic representationof the same cell, and FIG. 3c provides a cross section view along thechannel from source to drain. Transistor 300 consists of a split channelEprom transistor having a source 301, a drain 302, a floating gate 304controlling channel conduction along section L1 (312) of the channel, acontrol gate 309 capacitively coupled to the floating gate and alsocontrolling the conduction along the series portion of the channel L2(320), which has enhancement threshold voltage.

The transistor channel width (W), as well as the edges of the source anddrain diffusions are defined by the edges 305 of a thick field oxideformed by. isoplanar oxidation. Oxide 332 of thickness in the 25 to 40nanometers range is used as isolation between the floating gate and thesubstrate. Masuoka adds an erase gate 330 disposed underneath thefloating gate along one of its edges. This erase gate is used toelectrically erase floating gate 304 in an area of tunnel dielectric 331where the floating gate overlaps the erase gate. Tunnel dielectric 331is of thickness between 30 and 60 nanometers.

Masuoka specifies the following voltages during erase: V_(S) =0 V, V_(D)=0 V,V_(CG) =0 ,V_(BB) =0 , V_(ERASE) 20 V to 30 V.

Comparing the Masuoka cell with the Samachisa and Kynett cells:

(a) Masuoka's cell does not erase by using either the source diffusionor the drain diffusion for tunnel erase. Therefore these diffusionsnever experience a voltage higher than during Eprom programming. Thejunction avalanche breakdown and junction leakage problems therefore donot exist.

(b) Masuoka's cell uses a relatively thick tunnel dielectric andtherefore does not need to use thin tunnel dielectrics for erase.Therefore it is less susceptible to oxide pinholes introduced during themanufacturing cycle.

(c) Masuoka's cell does not have a "program disturb" problem becauseprogramming and tunnel erase involve two different mechanisms occuringat two different regions of the transistor.

(d) Masuoka's cell is not susceptible to the overerase condition becauseof the presence of the series enhancement transistor channel 320 (L2).

(e) Masuoka's cell requires a third layer of polysilicon, whichcomplicates the process as well as aggravates the surface topology.Because the erase gate consumes surface area over the field oxide 305 itresults in a larger cell.

(f) The overlap area 331 in Masuoka's cell is sensitive to maskmisalignment between the two masks defining this overlap. Since theoverlap area is nominally very small, even small misalignments canresult in large variations in the area used for tunnel erase. Thisresults in severe variations from wafer to wafer.

From the foregoing analysis it is clear that while the Masuoka prior artcell successfully addresses most of the problems encountered bySamachisa and Kynett, it itself has disadvantages not encountered bySamachisa or Kynett.

Masuoka and Samachisa both use a split channel Eprom transistor forprogramming. In the split channel eprom transistor, the portion L2 ofthe channel length controlled by control gate 109, 309 has a fixedenhancement threshold voltage determined by the p+ channel dopingconcentration 360. The portion L1 of the channel length controlled byfloating gate 104 (Samachisa) and 304 (Masuoka) has a variable thresholdvoltage determined by the net charge stored on the floating gate.

Other prior art split channel Eprom transistors are described by E.Harari in U.S. Pat. No. 4,328,565, May 4, 1982 and by B. Eitan in U.S.Pat. No. 4,639,893, Jan. 27, 1987. The Harari split channel Epromtransistor 300d is shown in cross section in FIG. 3d. Source 301d anddrain 302d are formed prior to formation of the floating gate 304d.Therefore, the total channel length L1+L2 is insensitive to maskmisalignment. However, both L1 and L2 are sensitive to misalignmentbetween floating gate 304d and drain diffusion 302d.

The Eitan split channel Eprom transistor 400 is shown in cross sectionsin FIG. 4a. The Eitan patent highlights the main reasons for using asplit channel architecture rather than the standard self aligned stackedgate Eprom transistor 200 (FIG. 2). These reasons can be summarized asfollows:

The addition of a fixed threshold enhancement transistor in series withthe floating gate transistor decouples the floating gate from the sourcediffusion. This allows the channel length L1 to be made very smallwithout encountering punchthrough between source and drain. Furthermore,transistor drain-turn on due to the parasitic capacitive couplingbetween the drain diffusion and the floating gate is eliminated becausethe enhancement channel portion L2 remains off.

Eitan shows that the shorter the length L1 the greater the programmingefficiency and the greater the read current of the split channel Epromtransistor. For Flash EEprom devices the series enhancement channel L2acquires additional importance because it allows the floating gateportion L1 to be overerased into depletion thereshold voltage withoutturning on the composite split channel transistor.

The disadvantages incurred by the addition of the series enhancementchannel L2 are an increase in cell area, a decrease in transistortransconductance, an increase in control gate capacitance, and anincrease in variability of device characteristics for programming andreading brought about by the fact that L1 or L2 or both are notprecisely controlled in the manufacturing process of the prior art splitchannel devices. Samachisa, Masuoka and Eitan each adopt a differentapproach to reduce the variability of L1 and L2:

Samachisa's transistor 100 (FIG. 1) uses the two edges 140, 143 ofcontrol gate 109 to define (by a self aligned ion implant) draindiffusion 102 and source diffusion 101. Edge 141 of floating gate 104 isetched prior to ion implant, using edge 140 of control gate 109 as anetch mask. This results in a split channel transistor where (L1+L2) isaccurately controlled by the length between the two edges 140, 143 ofthe control gate. However, L1 and L2 are both sensitive to misalignmentbetween the mask defining edge 142 and the mask defining edges 140, 143.

Masuoka's transistor 300 (FIG. 3c) forms both edges 341, 342 of floatinggate 304 in a single masking step. Therefore L1 is insensitive to maskmisalignment. L2, which is formed by ion implant of source diffusion 301to be self aligned to edge 343 of control gate 309, is sensitive tomisalignment between the mask defining edge 342 and the mask definingedge 343. Furthermore the Masuoka transistor 300 may-form a thirdchannel region, L3, if edge 340 of control gate 309 is misaligned in adirection away from edge 341 of floating gate 304 the formation of L3will severely degrade the programming efficiency of such a cell.

Eitan's transistor 400 (FIGS. 4a, 4b) uses a separate mask layer 480 toexpose the edge of floating gate 404 to allow drain diffusion 402 to beself aligned (by ion implantation) to edge 441 of floating gate 404.Therefore L1 can be accurately controlled and is not sensitive to maskmisalignment. L2 however is sensitive to the misalignment between edge482 of photoresist 480 and edge 442 of the floating gate. Eitan claimsthat the variability in L2 due to this mask misalignment, can be as muchas 1.0 micron or more without affecting the performance of the device(see claims 3, 4 of the above-referenced Eitan patent).

It should be pointed out that even with the most advanced opticallithography systems available today in a production environment it isdifficult to achieve an alignment accuracy of better than ±0.25 micronsbetween any two mask layers. Therefore the variability in L2 or L1inherent to any structure which is alignment sensitive can be as much asapproximately 0.5 microns from one extreme to the other.

Another prior art split channel Eprom device which attempts to achievethe objective of accurately establishing L1 and L2 is disclosed by Y.Mizutani and K. Makita in the 1985 IEDM Technical Digest, p. 63, shownin cross section in FIG. 4c. Transistor 400c has a floating gate 404cformed along the sidewall 440c of control gate 409c. In this way both L1and L2 can be independently established and are not sensitive to maskmisalignment. Transistor 400c has the drawback that the capacitivecoupling between control gate 409c and floating gate 404c is limited tothe capacitor area of the sidewall shared between them, which isrelatively a small area. Therefore there is a very weak capacitivecoupling between the control gate and the floating gate either duringprogramming or during read. Therefore, although the device achieves goodcontrol of L1 and L2 it is of rather low efficiency for both modes ofoperation.

Yet another prior art device which has a split channel with a wellcontrolled L1 and L2 is disclosed by A. T. Wu et al. in the 1986 IEDMTechnical Digest, p. 584 in an article entitled "A Novel High-Speed,5-Volt Programming Eprom Structure with Source-Side Injection". A crosssection of the Wu prior art transistor is shown in FIG. 4d (FIG. 2 inthe above-referenced article). This transistor has a floating gate 404dcoupled to a control gate 409d, extending over channel region L1 (412d),in series with a second floating gate 492d formed in a sidewall adjacentto source diffusion 401d and overlying channel region L2 (420d). Thissecond floating gate is capacitively coupled to the control gate 409dthrough the relatively small area of the sidewall 493d shared betweenthem, and is therefore only marginally better than the Mizutani priorart device, although it does achieve a good control of both L1 and L2.

Another prior art Eprom transistor which does not have a split channelstructure but which seeks to achieve two distinct channel regions tooptimize the Eprom programming performance is disclosed by S. Tanaka etal. in 1984 ISSCC Digest of Technical Papers, p. 148 in an articleentitled "A Programmable 256 K CMOS Eprom with On Chip Test Circuits". Across section of this device is shown in FIG. 4e (corresponding to FIG.3 in the Tanaka article). Transistor 400e is a stacked gate Epromtransistor (not split channel) with source 401e and drain 402e selfaligned to both edges of floating gate 404e and control gate 409e. Thechannel region is more heavily p doped 460e than the p substrate 463e,but in addition there is a second p+ region 477e which is even moreheavily p-doped than region 460e. This region 477e is formed bydiffusion of boron down and sideways from the top surface on the drainside only, and is formed after formation of the floating gate so as tobe self aligned to the floating gate on the drain side. The extent ofsideway diffusion of boron ahead of the sideway diffusion of arsenicfrom the N+ drain junction defines a channel region Lp (478e) adjacentto the drain. This is a DMOS type structure, called DSA (Diffusion SelfAligned) by Tanaka. The presence of the p+ region 478e reducesconsiderably the width of the drain depletion region during high voltageprogramming. A shorter depletion layer width results in greater energybeing imparted to channel electrons entering the depletion region, whichin turn results in significant increase in programming efficiencythrough hot electron injection. Transistor 400e has proven difficult tomanufacture because it is rather difficult to control the length Lp andthe surface channel concentration p+ through a double diffusion step.Furthermore, it is rather difficult to obtain value of Lp bigger thanapproximately 0.3 microns by diffusion because device scaling dictatesthe use of rather low temperature diffusion cycles. Still further, theDSA Eprom device suffers from an excessively high transistor thresholdvoltage in the unprogrammed (conducting) state, as well as from highdrain junction capacitance. Both these effects can increasesubstantially the read access time.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

I.a. Split Channel Eprom Transistor with Self Aligned Drain Diffusionand Self Aligned Spaced Apart Source Diffusion

FIG. 5a presents a cross sectional view of a split channel Epromtransistor in accordance with a first embodiment of this invention.Transistor 500a consists of a p type silicon substrate 563 (which canalternatively be a p type epitaxial layer grown on top of a p++ dopedsilicon substrate), N+ source diffusion 501a, N+ drain diffusion 502a, achannel region 560a which is more heavily p-doped than the surroundingsubstrate, a floating gate 504a overlying a portion L1 of the channel,512a, and a control gate 509 overlying the remaining portion L2 of thechannel, 520a as well as the floating gate. Floating gate 504a isdielectrically isolated from the surface of the silicon substrate bydielectric film 564a, which is thermally grown Silicon Dioxide. Controlgate 509 is capacitively coupled to floating gate 504a throughdielectric film 567a, which can either be thermally grown SiliconDioxide or a combination of thin layers of Silicon Dioxide and SiliconNitride. Control gate 509 is also insulated from the silicon surface inchannel portion L2 as well as over the source and drain diffusions bydielectric film 565a, which is made of the same material as dielectric567a.

P-type substrate 563 is typically 5 to 50 Ohms centimeter, p+ channeldoping 560a is typically in the range of 1×10¹⁶ cm³⁻³ to 2×10¹⁷ cm⁻³,dielectric film 564a is typically 20 to 40 nanometers thick, dielectricfilm 567a is typically 20 to 50 nanometers thick, floating gate 504a isusually a heavily N+ doped film of polysilicon of thickness which can beas low as 25 nanometers (this thickness will be discussed in SectionVII) or as high as 400 nanometers. Control gate 509 is either a heavilyN+ doped film of polysilicon or a low resistivity interconnect materialsuch as a silicide or a refractory metal. Of importance, edge 523a of N+drain diffusion 502a formed by ion implantation of Arsenic or Phosphorusis self aligned to edge 522a of floating gate 504a, while edge 521a ofN+ source diffusion 501a formed by the same ion implantation step isself aligned to, but is spaced apart from, edge 550a of the samefloating gate 504a, using a sidewall spacer (not shown in FIG. 5a) whichis removed after the ion implantation but prior to formation of controlgate 509. The implant dose used to form diffusions 501a, 502a, istypically in the range of 1×10¹⁵ cm⁻² to 1×10¹⁶ cm⁻².

The key steps for the formation of channel portions L1 and L2 areillustrated in FIGS. 5b through 5f. In the structure of FIG. 5b floatinggates 504a, 504b are formed in a layer of N+ doped polysilicon on top ofa thin gate oxide 564a, by anisotropic reactive ion etchings, usingphotoresist layer 590 as a mask. In FIG. 5c a thin protective film 566ais deposited or thermally grown, followed by the deposition of a thickspacer layer 570. The purpose of film 566a is to protect the underlyingstructure such as layer 565a from being etched or attacked when thespacer film is etched back. The spacer film is now etched back in ananisotropic reaction ion etch step with carefully controlled timing. Theconditions for etchback must have no significant undercutting and musthave a differential etch rate of 20:1 or higher between the spacermaterial and the material of protective film 566a. Spacer layer 570 canbe a conformal film of undoped LPCVD polysilicon while protective film566a can be silicon dioxide or silicon nitride. Alternatively, spacerlayer 570 can be a conformal film of LPCVD silicon dioxide whileprotective film 566a can be either LPCVD silicon nitride or LPCVDpolysilicon. The thickness of protective film 566a should be as thin aspossible, typically in the range of 10 to 30 nanometers, so as to toallow penetration of the subsequent Arsenic implantation to form thesource and drain diffusions.

The thickness of the conformal spacer layer determines the width of thesidewall spacer, and therefore also the length of channel portion L2.Typically for an L2 of 400 nanometers a spacer layer of approximately600 nanometers thickness is used.

In FIG. 5d spacers 592a, 593a and 592b, 593b are formed along thevertical edges of floating gates 504a and 504b respectively at thecompletion of the timed reactive ion etch step. These spacers resultfrom the fact that the thickness of layer 570 is greater adjacent to thevertical walls of the floating gates than it is on flat surfaces.Therefore a carefully timed anisotropic reactive ion etchback will etchthrough layer 570 in areas of flat surface topology while not completelyetching through it along each edge, forming the spacers. The techniquefor formation of narrow sidewall spacers along both edges of the gate ofMOS transistors is well known in the industry, and is commonly used toform lightly doped drains (LDD) in short channel MOSFETS. (See, forexample, FIG. 1 in an article in 1984 IEDM Technical Digest, p. 59 by S.Meguro et al. titled "Hi-CMOS III Technology".)

In the present invention, the spacer can be significantly wider, it isused along one edge only, and it is used not to define a lightly dopedsource or drain but rather to define the series enhancement transistorchannel portion L2.

The next step is a masking step. Photoresist 591a, 591b (FIG. 5d) isused as a mask to protect spacers 592a, 592b while exposing spacers593a, 593b. The latter are etched away, preferably with a wet chemicaletch (which should be chosen so as to not etch protective film 566a),and the photoresist is stripped.

In FIG. 5e ion implantation of Arsenic through dielectric films 566a and565a is used to form N+ source diffusions 501a, 501b and N+ draindiffusions 502a, 502b. On the drain side these diffusions are selfaligned to edges 522a and 522b of the floating gates. On the source sidethe diffusions are self aligned to edges 550a and 550b of the floatinggates but are spaced apart from these edges by the width of spacers 592aand 592b less the sideways diffusion in subsequent high temperatureprocess steps.

Next, spacers 592a, 592b and the protective film 566a are removed (FIG.5f), preferably with wet etches which will not attack the underlyinglayers 565a and 504a. Dielectric film 567a is grown by thermal oxidationor deposited by LPCVD on the exposed surfaces of the floating gates andsubstrate. A conductive layer is then deposited and control gates 509a,509b are formed through etching of long narrow strips which constitutethe word lines in rows of memory cells in an array.

The remaining part of the process is standard:

The surface of the structure is covered with a thick passivation layer568, usually phosphorous doped glass or a Borophosphosilicate glass(BPSG). This passivation is made to flow in a high temperature annealstep. Contact vias are etched (not shown in FIG. 5f) to allow electricalaccess to the source and drain diffusions. Metallic interconnect strips569a, 569b are provided on top of passivation layer 568, accessing thesource and drain diffusions through the via openings (not shown).

Comparing split channel transistor 500a of FIG. 5f with the Samachisa,Masuoka, Harari and Eitan prior art split channel transistors 100, 300,300d and 400, the advantages of transistor 500a can be summarized asfollows:

a) L1 and L2 are insensitive to mark misalignment. Therefore they can becontrolled much more accurately and reproducibly than the prior art.

b) Because all four prior art transistors 100, 300, 300d and 400 defineL2 through a mask alignment tolerance whereas transistor 500a defines L2through control of the width of a sidewall spacer it is possible intransistor 500a to achieve controllably a much shorter channel portionL2 than possible through a mask alignment. This becomes an importantconsideration in highly scaled split channel Eprom and Flash EEpromtransistors.

I.b. Split Channel Eprom Transistor with Heavily Doped Channel Adjacentto the Drain Junction

FIG. 14c presents a cross sectional view of a non self aligned splitchannel Eprom transistor in accordance with a second embodiment of thisinvention. FIGS. 14a and 14b illustrate the critical process steps inthe manufacturing process of this device. Transistor 1400 consists of ap type silicon substrate 1463 (which can also be a p type epitaxiallayer grown on a p++ substrate). Shallow N+ source diffusions 1401 andN+ drain diffusions 1402 are formed prior to formation of floating gate1404, in contrast with the embodiment of section Ia above. The channelregion between the source and drain diffusions is split into twoportions: a portion L1 (1412) which is lying directly underneath thefloating gate, and a portion L2 (1420) which is lying directlyunderneath the control gate 1409. The improvement over the Harari priorart split channel transistor 300d (FIG. 3d) consists of a heavily p+doped narrow region 1460 adjacent to drain diffusion 1402. The width Lp(1413) and doping concentration of this region at the top surface wherethe field effect transistor channel is formed, become the controllingparameters for device programming and reading efficiency, provided thatp+ is sufficiently high. Typically, p substrate 1463 may have a p typedoping concentration of 1×10¹⁶ cm^(-') whereas p+ region 1460 may have ap+ type doping concentration of between 1×10¹⁷ cm⁻³ and 1×10¹⁸ cm⁻³. Inthe preferred manufacturing process the length Lp and dopingconcentration of region 1460 are chosen so that the depletion regionwidth at the drain junction under programming voltage conditions is lessthan the width Lp. So long as that condition is satisfied, and so longas L1 is bigger than Lp, then the actual value of L1 is of secondaryimportance to the device performance. Since L1 in this device isdetermined through a mask alignment between the floating gate and thedrain it is not as well controlled as in the Eitan prior art transistor400. However, to the extent that region 1460 can be made to be selfaligned to the drain so that parameter Lp is not sensitive to maskalignment, then any variability in L1 is of secondary importance, Lpbeing the controlling parameter.

A new method is disclosed for manufacturing the split channel Epromtransistor 1400 which results in much better control of the parameter Lpand of the surface channel doping concentration 1413 than is provided bythe DSA (Diffusion Self Align) approach of the Tanaka prior arttransistor 400e (FIG. 4e).

The main steps in this new method for the fabrication of a memory arrayof transistors 1400 are as follows:

1. In the structure of FIG. 14a a thin oxide layer 1475, typically 50nanometers of silicon dioxide, is covered with a layer 1474 of siliconnitride, approximately 100 nanometers thick. This in turn is coveredwith a second layer 1473 of deposited silicon dioxide, approximately 100nanometers thick. Oxide 1475 and nitride 1474 can, for example, be thesame films used to form isoplanar isolation regions in the periphery ofthe memory array.

2. A photoresist mask P.R.1 (1470 ) is used to define source and drainregions in long parallel strips extending in width between edges 1471,1472 of openings in the photoresist. Exposed oxide layer 1473 is now wetetched in a carefully controlled and timed etch step which includessubstantial undercutting of photoresist 1470. The extent ofundercutting, which is measured by the distance Lx between oxide edges1476 and 1478, will eventually determine the magnitude of parameter Lp.Typically, Lx is chosen between 300 nanometers and 700 nanometers. Thethree parameters critical for a reproducible Lx are the concentrationand temperature of the etch solution (hydrofluoric acid) and the density(i.e., lack of porosity) of the oxide 1473 being etched. These can bewell controlled sufficiently so that a timed undercutting etch stepresults in well controlled etched strips of width Lx and runningparallel to edges 1471, 1472 of the long openings in the photoresist. Infact, for values of Lx below approximately 500 nanometers, it is easierto achieve a reproducible Lx through controlled sideway etching than bycontrolling the line width of long, narrow line in a photoresist layer.An example of the use of sideway etching self aligned to an edge in asimilar fashion (but to achieve the different purpose of forming a verynarrow guard ring) can be found in the prior art article by S. Kimtitled "A Very Small Schottky Barrier Diode with Self-Aligned Guard Ringfor VLSI Application", appearing in the 1979 IEDM Technical Digest, p.49.

3. At the completion of the sideway etch step a second, anisotropic etchis performed, using the same photoresist mask P.R.1 to etch away longstrips of the exposed silicon nitride film 1474. Edges 1471, 1472 ofP.R.1 (1470 ) are used to form edges 1480, 1481 respectively in theetched strips of nitride layers.

4. Arsenic ion implantation with an ion dose of approximately 5×10¹⁵cm⁻² is performed with an energy sufficient to penetrate oxide film 1475and dope the surface in long strips of N+ doped regions (1402, 1401).Photoresist mask P.R.1 can be used as the mask for this step, butnitride layer 1474 can serve equally well as the implant mask. P.R.1 isstripped at the completion of this step.

5. An implant damage anneal and surface oxidation step follows,resulting in 200 to 300 nanometers of silicon dioxide 1462 grown overthe source and drain diffusion strips. The temperature for thisoxidation should be below 1000° C. to minimize the lateral diffusions ofthe N+ dopants in regions 1402, 1401. If desired it is possible throughan extra masking step to remove nitride layer 1474 also from the fieldregions between adjacent channels, so as to grow oxide film 1462 notonly over the source and drain regions but also over the field isolationregions.

6. In FIG. 14b a second photoresist mask P.R.2 (1482) is used to protectthe source-side (1401) of the substrate during the subsequent implantstep. This implant of boron can be performed at relatively high energysufficient to penetrate through nitride layer 1474 and oxide layer 1475but not high enough to penetrate top oxide 1473, nitride 1474 and oxide1475. Alternatively, nitride layer 1474 can first be etched along edge1482, using edge 1478 of the top oxide 1473 as a mask. The boron implantdose is in the range of 1×10⁻⁻ cm⁻² and 1×10¹⁴ cm⁻². The surface area ofheavy p+ doping 1460 is confined to the very narrow and long strip ofwidth extending between edge 1478 of the top oxide and the edge of theN+ diffusion 1402, and running the length of the drain diffusion strip.Note that the thick oxide 1462 prevents penetration of the boron implantinto the drain diffusion strip. This greatly reduces the drain junctioncapacitance, which is highly desirable for fast reading. Note also thatp+ region 1460 is automatically self aligned to drain region 1402through this process.

7. Top oxide 1473, nitride 1474 and thin oxide 1475 are now removed byetching. This etching also reduces the thickness of the oxide layer 1462protecting the source and drain diffusions. It is desirable to leavethis film thickness at not less than approximately 100 nanometers at thecompletion of this etch step.

8. The remaining steps can be understood in relation to the structure ofFIG. 14c: A gate oxide 1464 is grown over the surface, including thechannel regions, separating between the long source/drain diffusionstrips (typical oxide thickness between 15 and 40 nanometers). A layerof polysilicon is deposited (thickness between 25 and 400 nanometers),doped N+, masked and etched to form continuous narrow strips of floatinggates 1404 mask aligned to run parallel to drain diffusion strips 1402and to overlap p+ regions 1460.

9. A second dielectric 1466, 1411 is grown or deposited on top of thesubstrate and floating gate strips, respectively. This can be a layer ofsilicon dioxide or a combination of thin films of silicon dioxide andsilicon nitride, of combined thickness in the range between 20 and 50nanometers.

10. A second layer of polysilicon is deposited, doped N+ (or silicidedfor lower resistivity), masked and etched to form control gates 1409 inlong strips running perpendicular to the strips of floating gates andsource/drain strips. Each control gate strip is capacitively coupled tothe floating gate strips it crosses over through dielectric film 1411 inthe areas where the strips overlap each other. Control gates 1409 alsocontrol the channel conduction in channel portions L2 not covered by thefloating gate strips. Each strip of control gates is now covered by adielectric isolation film (can be thermally grown oxide).

11. Using the strips of control gates as a mask, exposed areas ofdielectric 1466, 1411 and of the strips of first polysilicon floatinggates are etched away. The resulting structure has long strips, or rows,of control gates, each row overlying several floating gates 1404 wherethe outer edges of each floating gate are essentially self aligned tothe edges defining the width of the control gate strip. These edges arenow oxidized or covered with a deposited dielectric to completelyinsulate each floating gate. Field areas between adjacent rows of cellsor between adjacent strips of source and drain regions are nowautomatically self aligned to the active device areas and do not requirespace consuming isoplanar oxidation isolation regions. (Of course, it isalso possible to fabricate transistor 1400 with source, drain andchannel regions defined by the edges of a thick isoplanar oxidationisolation layer, or to rely for field isolation on oxide 1462 grown alsoin the field regions, see the option described in step 5 above.)

The Eprom cell of this embodiment has several advantages over the priorart Eprom cells:

a) Control gate 1409 now runs over a relatively thick oxide 1462 overthe source and drain regions. Such a thick oxide is not possible forexample with the prior art Eitan cell, where these source and drainregions are formed after, not before, the floating gate is formed. Thisimproves the protection from oxide breakdowns and reduces the parasiticcapacitance between control gate and drain.

b) Control of parameter Lp and of the surface P+ doping concentration inregion 1460 is superior to that afforded by the DSA prior art Tanakacell.

c) The device sensitivity to misalignment between floating gate anddrain is far less than that experienced with the prior art Harari,Samachisa and Masuoka cells.

d) For a given p+ concentration in the channel region, drain junctioncapacitance is less with this cell than with all other prior artdevices, because p+ region 1460 is very narrowly confined near the draindiffusion.

e) It is possible to dope p+ region 1460 to very high levels (whichsignificantly enhances the programming efficiency) without undulyraising the conduction threshold voltage in the enhancement serieschannel region L2. This is particularly useful for Flash EEpromembodiments using this cell for the Eprom part. In such a Flash EEprom,the high initial threshold voltage in region Lp controlled by floatinggate 1404 (initial Vt can be as high as +5.0 V, the supply voltage, orhigher), can be easily overcome by erasing the cell to lower thresholdvoltages. As an Eprom device the initial Vt in the unprogrammed statemust not be higher than the control gate voltage during read, and thisrequirement sets an upper limit on how high the p+ doping concentrationcan be. Another limit on the magnitude of p+ doping concentration 1460is established by the minimum drain voltage necessary for programming.The drain junction avalanche breakdown voltage must be at least as highas this minimum programming voltage.

II. Self Aligned Split Channel Flash EEprom Cell With Isoplaner FieldIsolation

FIG. 6a presents a topological view of a 2×2 memory array consisting offour Flash EEprom transistors 600a, 600b, 600c and 600d in accordancewith one embodiment of this invention. FIG. 6b presents a cross sectionview of the same structure along AA of FIG. 6a. A second cross sectionalong BB results in the Eprom transistor 500a shown in FIG. 5a.

Transistor 600a of FIG. 6a is a split channel Eprom transistor which hasadded to it erase gates 530, 535, which overlap edges 532a, 562a offloating gate 504a. Transistor 600a is programmed as a split channelEprom transistor having a source diffusion 501a, a drain diffusion 502a,and a control gate 509. Floating gate 504a and channel portions L1 andL2 are formed in accordance with the split channel Eprom transistor 500aof section I.a. or the split channel Eprom transistor 1400 of sectionI.b. However other split channel Eprom devices (such as the Eitan,Harari, Masuoka or Samachisa prior art Eprom) can also be used for theEprom structure. The transistor channel width W is defined by the edges505, 505a of a thick field oxide 562.

Transistor 600a is erased by tunneling of electrons from floating gate504a to erase gates 530, 535, across tunnel dielectrics 531a, 561a onthe sidewalls and top surface of the floating gate where it isoverlapped by the erase gate.

Tunnel dielectric film 531a, 561a is normally a layer of Silicon Dioxidegrown through thermal oxidation of the heavily N+ doped and texturedpolycrystalline silicon comprising the floating gate. It is well knownin the industry (see for example an article by H.A.R. Wegener titled"Endurance Model for textured-poly floating gate memories", TechnicalDigest of the IEEE International Electron Device Meeting, Dec. 1984, p.480) that such a film, when grown under the appropriate oxidationconditions over properly textured doped polysilicon allows an increaseby several orders of magnitude of the conduction by electron tunnelingeven when the film is several times thicker than tunnel dielectric filmsgrown on single crystal silicon (such as the tunnel dielectric filmsused in the prior art Samachisa and Kynett devices). For example, atunnel dielectric oxide grown to a thickness of 40 nanometers on N+doped and textured polysilicon can conduct by electronic tunnelingapproximately the same current density as a tunnel dielectric oxide of10 nanometers thickness grown on N+ doped single crystal silicon underidentical voltage bias conditions. It is believed that this highlyefficient tunneling mechanism is a result of sharp asperities at thegrain boundaries of the polysilicon which is specially textured toenhance the areal density of such asperities. A commonly practicestechnique is to first oxidize the surface of the polysilicon at a hightemperature to accentuate the texturing, then stripping that oxide andregrowing a tunnel oxide at a lower temperature. The oxide film cappingsuch an asperity experiences a local amplification by a factor of fourto five of the applied electric field resulting in an efficientlocalized tunnel injector. The advantage provided by the thicker filmsof tunnel dielectric is that they are much easier to grow in uniform anddefect-free layers. Furthermore the electric field stress duringtunneling in the thick (40 nanometer) tunnel dielectric is only 25percent of the stress in the thin (10 nanometer) tunnel dielectric,assuming the same voltage bias conditions. This reduced stresstranslates into higher reliability and greater endurance to write/erasecycling. For these reasons, all Flash EEprom embodiments of thisinvention rely on poly--poly erase through a relatively thick tunneldielectric.

In the embodiment of FIGS. 6a, 6b floating gate 504a is formed in afirst layer of heavily N+ doped polysilicon of thickness between 25 and400 nanometers, erase gates 530, 535 are formed in a second layer of N+doped polysilicon of thickness between 50 and 300 nanometers, andcontrol gate 509 is formed in a third conductive layer of thicknessbetween 200 and 500 nanometers, which may be N+ doped polysilicon or apolycide, a silicide, or a refractory metal. The erase gate can beformed in a relatively thin layer because a relatively high sheetresistivity (e.g., 100 Ohm per square) can be tolerated since almost nocurrent is carried in this gate during tunnel erase.

The manufacturing process can be somewhat simplified by implementingerase gates 530, 535 in the same conductive layer as that used forcontrol gate 509. However the spacing Z between the edges of the controlgate and the erase gate (and hence the cell size) would then have to besignificantly greater than is the case when the control gate and erasegates are implemented in two different conductive layers insulated fromeach other by dielectric film 567a. In fact, in the triple layerstructure 600a of FIG. 6a it is even possible to have control gate 509slightly overlap one or both of the erase gates 530 and 535 (i.e.,spacing Z can be zero or negative.) Transistor 600a employs a fieldisolation oxide 562 (FIG. 6b) of thickness between 200 and 1000nanometers. Gate oxide 564a protecting channel portion L1 (512a) isthermally grown silicon dioxide of thickness between 15 and 40nanometers. Dielectric film 567a which serves to strongly capacitivelycouple control gate 509 and floating gate 504a is grown or deposited. Itmay be silicon dioxide or a combination of thin films of silicon dioxideand oxidized silicon nitride of combined thickness of between 20 and 50nanometers. This dielectric also serves as part of the gate oxideprotecting channel portion L2 (520a) as well as insulation 565a (FIG.5a) over the source and drain diffusions. Erase dielectric 531a, 561a isthermally grown Silicon Dioxide or other deposited dielectricspossessing the appropriate characteristics for efficient eraseconduction, such as Silicon Nitride. Its thickness is between 30 and 60nanometers.

A point of significance is the fact that the tunnel dielectric areacontributing to erase in each cell consisting of the combined areas of531a and 561a, is insensitive to the mask misalignment between edges532a, 562a of floating gate 504a and erase gates 530, 535. (Note thateach erase gate, such as 535, is shared between two adjacent cells, suchas 600a and 600c in this case). Any such misalignment will result in areduction of the area of the tunnel dielectric at one edge of thefloating gate, but also in an increase of equal magnitude in the areaavailable for tunneling at the other edge of the floating gate. Thisfeature permits the construction of a cell with very small area oftunnel dielectric. By contrast the prior art triple layer Flash EEpromcells of Masuoka and Kuo referenced above are sensitive to maskmisalignment and therefore require a structure wherein the nominal areaprovided for tunnel erase may be much larger than the optimum such area,in order to accommodate for the worst case misalignment condition.

Another distinguishing feature of this embodiment relative to theMasuoka cell of FIGS. 3a and 3b is that Masuoka implements the erasegate in a first conductive layer 330 and the floating gate in a secondconductive layer 304, i.e., in a reverse order to that used in thisinvention. This results in a far less efficient tunnel erase in theMasuoka cell because the asperities in Masuoka's tunnel dielectric 331are at the surface of the erase gate (collector) rather than at theinjecting surface of the floating gate. Therefore Masuoka's cellrequires higher electric fields (and therefore higher V_(ERASE)voltages) than the structure of this invention.

Typical bias voltage conditions necessary to erase memory cells 600a,600b, 600 c and 600d are:

V_(ERASE) (on all erase gates 530, 535, 536)=15 V to 25 V applied forbetween 100 milliseconds and 10 seconds (the pulse duration is stronglydependent on the magnitude of V_(ERASE)) V_(CG) =0 V, V_(BB) =0 V. V_(D)and V_(S) can be held at 0 V or at a higher voltage between 5 V and 10V, so as to reduce the net voltage experienced during erase acrossdielectric film 565a in areas such as 563 (FIG. 6a) where erase gate 530crosses over drain diffusion 502.

III. Self Aligned Split Channel Flash EEprom Cell With Field PlateIsolation

A 2×2 array of Flash EEprom cells in accordance with another embodimentof this invention is shown in; topological view in FIG. 7a and in twocross sectional views AA and CC in FIGS. 7b and 7c respectively. Crosssectional view BB is essentially the same as the split channel Epromtransistor of FIG. 5a.

Split channel Flash EEprom transistor 700a employs three conductivelayers (floating gate 704 erase gates 730, 735 and control gate 709)formed in the same sequence as described in section II in conjunctionwith the Flash EEprom transistor 600a of FIGS. 6a, 6b. The majordistinguishing feature of transistor 700a is that erase gates 730, 735,736 are used not only for tunnel erase but also as the switched offgates of isolation field transistors formed outside the activetransistor regions. Thus, the thick isoplaner isolation oxide 562 ofcell 600a (FIG. 6b) is not necessary, and is replaced inside the arrayof memory cells 700a, 700b, 700c and 700d by a much thinner oxide 762(FIGS. 7b, 7c) capped with field plates 730, 735, 736, which are held at0 V at all times except during erasing.

The elimination of the thick isoplanar oxide inside the array of memorycells (this isoplanar oxide may still be retained for isolation betweenperipheral logic transistors) has several advantages:

1. The surface stress at the silicon--silicon dioxide boundary due to aprolonged thermal isoplanar oxidation cycle is eliminated inside thearray, resulting in less leaky source and drain junctions and in higherquality gate oxides.

2. For a given cell width, the elimination of the isoplanar oxide allowsthe effective channel width W₁ under floating gate 704 to extend all theway between the two edges 732a, 762a of the floating gate. Bycomparison, effective channel width W of transistor 600a (FIG. 6b) isdetermined by the edges 505 of the isoplanar oxide and is thereforesubstantially smaller. This difference results in a higher read signalfor cell 700a, or a narrower, smaller cell.

3. From capacitive coupling considerations (to be discussed in sectionVI below) the efficiency of tunnel erase is higher in cells wherecoupling of the floating gate to the silicon substrate 763 is greatest.In transistor 700a the entire bottom surface area of the floating gateis tightly coupled to the substrate 763 through the thin gate dielectric764. By contrast, in transistor 600a (FIG. 6b) much of the bottomsurface area of floating gate 504a overlies the thick field oxide 562and is therefore not strongly capacitively coupled to substrate 563.

4. The width of control gate 709 between its edges 744 and 774 defineschannel width W₂ of the series enhancement channel portion L2 (FIG. 7c).This permits the reduction in overall cell width due to removal of therequirement for the control gate to overlap the edges of the isoplaneroxide. One precaution necessary in the fabrication of cell 700a is thatany misalignment between the mask layers defining edge 732a of floatinggate 704a, edge 784 of erase gate 730, and edge 744 of control gate 709must not be allowed to create a situation where a narrow parasitic edgetransistor is created under control gate 709 in parallel with the splitchannel L1 and L2. However, as with cell 600a, since erase gates 730,736 and control gate 709 are formed in two separate conductive layerswhich are isolated from each other by dielectric insulator film 767(FIG. 7b) there is no requirement placed on the magnitude of the spatialseparation Z between edge 784 and edge 744. In fact, the two edges canbe allowed to overlap each other through oversizing or throughmisalignment, i.e., Z can be zero or negative. Dielectric insulator 767also forms part of the gate dielectric 766 (FIG. 7c) over channelportion L2.

In a memory array source diffusion 701 and drain diffusion 702 can beformed in long strips. If transistor 500a is used as the Epromtransistor, then source diffusion edge 721 is self aligned to thepreviously discussed sidewall spacer (not shown) while drain diffusionedge 723 is self aligned to edge 722 of floating gate 704a. In areasbetween adjacent floating gates 704a, 704c the source and draindiffusion edges (721x, 723x in FIG. 7a) respectively must be preventedfrom merging with one another. This can be accomplished by for examplefirst forming floating gates 704a, 704c as part of a long continuousstrip of polysilicon, then using this strip with an associated longcontinuous strip of sidewall spacer to form by ion implantation longdiffusion strips 701, 702, removing the spacer strip, and only theeetching the long continuous strip of polysilicon along edges 732a, 762ato form isolated floating gates 704a, 704c. As with the prior FlashEEprom embodiment it is possible to form this embodiment also inconjunction with Eprom cell 1400 (FIG. 14c) or with any other prior artsplit channel Eproms so long as they do not have their isoplanarisolation oxide inside the memory array.

IV. Self Aligned Split Channel Flash EEprom Cell with Erase Confined toThe Vertical Edges of The Floating Gate.

Another embodiment of the self aligned split channel Flash EEprom ofthis invention can result in a cell which has smaller area than cells600a and 700a of the embodiments described in Sections II and IIIrespectively. In this third embodiment the area for tunnel erase betweenthe floating gate and the erase gate is confined essentially to thesurfaces of the vertical sidewalls along the two edges of each floatinggate. To best understand how cell 800a of this embodiment differs fromcell 700a a 2×2 array of cells 800a, 800b, 800c and 800d are shown inFIG. 8a in topological view and in FIG. 8b along the same cross sectiondirection AA as is the case in FIG. 7b for cells 700a, 700c.

Cell 800a has a floating gate 804a formed in a first layer of heavily N+doped polysilicon. This gate controls the transistor conduction inchannel portion L1 (FIG. 8a) through gate oxide insulation film 864.Control gate 809 is formed in the second conductive layer, and isinsulated from the floating gate by dielectric film 867, which may be athermally grown oxide or a combination of thin silicon dioxide andsilicon nitride films. Edges 874, 844 of control gate 809 are used as amask to define by self aligned etching the edges 862a, 832a respectivelyof floating gate 804a. Erase gates 830, 835 are formed in a thirdconductive layer and are made to overlap edges 832a, 862a of floatinggate 804a. Each erase gate such as 830 is shared by two adjacent cells(such as 800a, 800c).

The erase gates are insulated from control gate 809 by dielectricinsulator 897 which is grown or deposited prior to deposition of erasegates 830, 835, 836. Tunnel erase dielectrics 831a, 861a are confined tothe surface of the vertical edges 832a, 862a of the floating gate 804a.Erase gate 830 also provides a field plate isolation over oxide 862 inthe field between adjacent devices.

The thickness of all conducting and insulating layers in structure 800are approximately the same as those used in structure 700a. However,because the erase gate is implemented here after, rather than before thecontrol gate, the fabrication process sequence is somewhat different.Specifically (see FIGS. 8a, 8b):

1. Floating gates 804a, 804c are formed in long continuous and narrowstrips on top of gate oxide 864. The width of each such strip is L1 plusthe extent of overlap of the floating gate over the drain diffusion.

2. Dielectric 867 is formed and the second conductive layer (N+ dopedpolysilicon or a silicide) is deposited.

3. Control gates 809 are defined in long narrow strips in a directionperpendicular to the direction of the strips of floating gates. Thestrips are etched along edges 844, 874, and insulated with relativelythick dielectric 897.

4. Edges 844, 874 (or the edges of insulator spacer 899 formed at bothedges of control gate strip 809) are then used to etch dielectric 867and then, in a self aligned manner to also etch vertical edges 832a and862a of the underlying floating gate strips, resulting in isolatedfloating gates which have exposed edges of polysilicon only along thesevertical walls.

5. Tunnel dielectric films 831a, 861a are formed by thermal oxidation ofthese exposed surfaces.

6. A third conductive layer is deposited, from which are formed erasegates 830 in long strips running in between and parallel to adjacentstrips of control gates. These erase gates also serve as field isolationplates to electrically isolate between adjacent regions in the memoryarray.

Flash EEprom transistor 800a can be implemented in conjunction with anyof the split channel Eprom transistors of this invention (transistors500a and 1400) or with any of the prior art split gate Eprom transistorsof Eitan, Samachisa, Masuoka or Harari. For example, an array of FlashEEprom transistors 800a can be fabricated by adding a few process stepsto the fabrication process for the split channel Eprom transistor 1400(FIG. 14c), as follows:

Steps 1 through 10 are identical to steps 1 through 10 described inSection I.b. in conjunction with the manufacturing process for splitchannel Eprom transistor 1400.

Steps 11, 12, and 13 are the process steps 4, 5, and 6 respectivelydescribed in this section IV in conjunction with split channel FlashEEprom transistor 800a.

Cell 800 a results in a very small area of tunnel erase, which is alsorelatively easy to control (it is not defined by a mask dimension, butrather by the thickness of the deposited layer constituting the floatinggates). For this reason, this cell is the most highly scalableembodiment of this invention.

V. Self Aligned Split Channel Flash EEprom Cell With a Buried EraseGate.

A 2×2 array of Flash EEprom cells 900a, 900b, 900c and 900d inaccordance with a fourth embodiment of this invention is shown intopological view in FIG. 9a and in two cross sectional views AA and DDin FIGS. 9b and 9c respectively. Cross section BB of FIG. 9a yields thesplit channel Eprom structure 500a of FIG. 5a.

Transistor 900a is a split channel Flash EEprom transistor havingchannel portions L1 and L2 formed by self alignment as in Epromtransistor 500a or in a non self aligned manner as in Eprom transistor1400. Erase gate 930 is a narrow conductive strip sandwiched betweenfloating gate 904a on the bottom and control gate 909 on top. Erase gate930 is located away from edges 932a, 962a of the floating gate. Theseedges therefore play no role in the tunnel erase, which takes placethrough tunnel dielectric 931 confined to the area where erase gate 930overlaps floating gate 904a. Erase gate 930 also overlaps a width W_(e)of the series enhancement channel portion L2. During read orprogramming, erase gate 930 is held at 0 V, and therefore the channelportion of width W.sub.ε does not contribute to the read or programcurrent. The only contribution to conduction in channel portion L2 comesfrom widths W_(p) and W_(q) where the channel is controlled directly bycontrol gate 909. Channel portion L1 however sees conductioncontributions from all three widths, W_(p), W_(q) and W_(e). Edges 932a,962a of floating gate 904a can be etched to be self aligned to edges944, 974 respectively of control gate 909. This then permits theformation of channel stop field isolation 998, by implanting a p typedopant in the field regions not protected by the control gate orfloating gate (FIG. 9b).

One advantage of cell 900a is that erase gate strips 930, 936 can bemade very narrow by taking advantage of controlled undercutting by forexample isotropic etchings of the conductive layer forming these strips.This results in a small area of tunnel erase, which is insensitive tomask misalignment. Furthermore the channel width W_(p) and W_(q) is alsoinsensitive to mask misalignment. This embodiment of Flash EEprom canalso be implemented in conjunction with prior art spilt channel Epromscells such as the Eitan, Harari, Samachisa or Masuoka cells.

VI. Device Optimization

FIG. 10 represents a schematic of the major capacitances which couplethe floating gate of the split channel Flash EEprom cells of thisinvention to the surrounding electrodes.

Specifically these are:

C_(G) = Capacitance between Floating gate 1104 and control gate 1109.

C_(D) = Capacitance between Floating gate 1104 and drain diffusion 1102.

C_(B) = Capacitance between Floating gate 1104 and substrate 1163.

C_(E) = Capacitance between Floating gate 1104 and erase gate 1130.

C_(T) = C_(G) +C_(D) +C_(B) +C_(E) is the total capacitance. Q is thenet charge stored on the floating gate. In a virgin device, Q=0. In aprogrammed device Q is negative (excess electrons) and in an eraseddevice Q is positive (excess holes).

The voltage V_(FG) on Floating gate 1104 is proportional to voltagesV_(CG), V_(ERASE), V_(D), V_(BB) and to the charge Q according to thefollowing equation: ##EQU1##

In all prior art Eprom and Flash EEprom devices as well as in embodiment600a of this invention, the dominant factor in C_(T) is C_(G), thecoupling to the control gate. However, in embodiments 700a, 800a and900a C_(B) is also a major contributor by virtue of the fact that theentire bottom surface of the floating gate is strongly coupled to thesubstrate.

a. Electrical Erase

During erase, the typical voltage conditions are V_(CG) =0 V, V_(D) =0V, V_(S) =0 V, V_(BB) =0 V and V_(ERASE) =20 V. Therefore, substitutingin equation(1),

    V.sub.FG =Q/C.sub.T +20 C.sub.E /C.sub.T                   (2)

The electric field for tunnel erase is given by

    E.sub.ERASE =V.sub.ERASE /t-V.sub.FG /t                    (3)

where t is the thickness of the tunnel dielectric. For a givenV_(ERASE), E_(ERASE) is maximized by making V_(FG) small, which, fromequation (2) is possible if C_(E) /C_(T) is small. Embodiments 700a,800a and 900a allow this condition to be readily met: C_(E) is smallsince the area of tunnel dielectric is small, and C_(T) is large becauseboth C_(G) and C_(B) are large. These embodiments are thereforeparticularly well suited for efficiently coupling the erase voltageacross the tunnel dielectric.

b. Multistate storage

The split channel Flash EEprom device can be viewed as a compositetransistor consisting of two transistors T1 and T2 in series--FIG. 11a.Transistor T1 is a floating gate transistor of effective channel lengthL1 and having a variable threshold voltage V_(T1). Transistor T2 has afixed (enhancement) threshold voltage V_(T2) and an effective channellength L2. The Eprom programming characteristics of the compositetransistor are shown in curve (a) of FIG. 11b. The programmed thresholdvoltage V_(tx) is plotted as a function of the time t during which theprogramming conditions are applied. These programming conditionstypically are V_(CG) =12 V, V_(D) =9 V, V_(S) =V_(BB) =0 V. Noprogramming can occur if either one of V_(CG) or V_(D) is at 0 V. Avirgin (unprogrammed, unerased) device has V_(T1) =+1.5 V and V_(T2)=+1.0 V. After programming for approximately 100 microseconds the devicereaches a threshold voltage V_(tx) ≧+6.0 volts. This represents the off("0") state because the composite device does not conduct at V_(CG)=+5.0 V. Prior art devices employ a so called "intelligent programming"algorithm whereby programming pulses are applied, each of typically 100microseconds to 1 millisecond duration, followed by a sensing (read)operation. Pulses are applied until the device is sensed to be fully inthe off state, and then one to three more programming pulses are appliedto ensure solid programmability.

Prior art split channel Flash EEprom devices erase with a single pulseof sufficient voltage V_(ERASE) and sufficient duration to ensure thatV_(T1) is erased to a voltage below V_(T2) (curve b) in FIG. 11b).Although the floating gate transistor may continue to erase intodepletion mode operation (line (C) in FIG. 11b), the presence of theseries T2 transistor obscures this depletion threshold voltage.Therefore the erased on ("1") state is represented by the thresholdvoltage V_(tx) =V_(T2) =+1.0 V. The memory storage "window" is given byΔV=V_(tx) ("0")-V_(tx) ("1")=6.0-1.0=5.0 V. However, the true memorystorage window should be represented by the full swing of V_(tx) fortransistor T1. For example, if T1 is erased into depletion thresholdvoltage V_(T1) =-3.0 V, then the true window should be given by ΔV=6.0-(-3.0)=9.0 V. None of the prior art Flash EEprom devices take advantageof the true memory window. In fact they ignore altogether the region ofdevice operation (hatched region D in FIG. 11b) where V_(T1) is morenegative than VT₂.

This invention proposes for the first time a scheme to take advantage ofthe full memory window. This is done by using the wider memory window tostore more than two binary states and therefore more than a single bitper cell. For example, it is possible to store 4, rather than 2 statesper cell, with these states having the following threshold voltage:

State "3": -V_(T1) =-3.0 V, V_(T2) =+1.0 V (highest conduction)=1,1.

State "2": -V_(T1) =-0.5 V, V_(T2) =+1.0 V (intermediate conduction)=1,0.

State "1": -V_(T1) =+2.0 V, V_(T2) =+1.0 V (lower conduction)=0, 1.

State "0": -V_(T1) =+4.5 V, V_(T2) =+1.0 V (no conduction)=0, 0.

To sense any one of these four states, the control gate is raised toV_(CG) =+5.0 V and the source-drain current I_(DS) is sensed through thecomposite device. Since V_(T2) =+1.0 V for all four threshold statestransistor T₂ behaves simply as a series resistor. The conductioncurrent I_(DS) of the composite transistor for all 4 states is shown asa function of V_(CG) in FIG. 11c. A current sensing amplifier is capableof easily distinguishing between these four conduction states. Themaximum number of states which is realistically feasible is influencedby the noise sensitivity of the sense amplifier as well as by any chargeloss which can be expected over time at elevated temperatures. Eightdistinct conduction states are necessary for 3 bit storage per cell, and16 distinct conduction states are required for 4 bit storage per cell.

Multistate memory cells have previously been proposed in conjunctionwith ROM (Read Only Memory) devices and DRAM (Dynamic Random AccessMemory). In ROM, each storage transistor can have one of several fixedconduction states by having different channel ion implant doses toestablish more than two permanent threshold voltage states.Alternatively, more than two conduction states per ROM cell can beachieved by establishing with two photolithographic masks one of severalvalues of transistor channel width or transistor channel length. Forexample, each transistor in a ROM array may be fabricated with one oftwo channel widths and with one of two channel lengths, resulting infour distinct combinations of channel width and length, and therefore infour distinct conductive states. Prior art multistate DRAM cells havealso been proposed where each cell in the array is physically identicalto all other cells. However, the charge stored at the capacitor of eachcell may be quantized, resulting in several distinct read signal levels.An example of such prior art multistate DRAm storage is described inIEEE Journal of Solid-State Circuits, Feb. 1988, p. 27 in an article byM. Horiguchi et al. entitled "An Experimental Large-CapacitySemiconductor File Memory Using 16-Levels/Cell Storage". A secondexample of prior art multistate DRAM is provided in IEEE CustomIntegrated Circuits Conference, May 1988, p. 4.4.1 in an articleentitled "An Experimental 2-Bit/Cell Storage DRAM for Macrocell orMemory-on-Logic Applications" by T. Furuyama et al.

To take full advantage of multistate storage in Eproms it is necessarythat the programming algorithm allow programming of the device into anyone of several conduction states. First it is required that the devicebe erased to a voltage V_(T1) more negative than the "3" state (-3.0 Vin this example). Then the device is programmed in a short programmingpulse, typically one to ten microseconds in duration. Programmingconditions are selected such that no single pulse can shift the devicethreshold by more than one half of the threshold voltage differencebetween two successive states. The device is then sensed by comparingits conduction current I_(DS) with that of a reference current sourceI_(REF), i (i=0,1,2,3) corresponding to the desired conduction state(four distinct reference levels must be provided corresponding to thefour states). Programming pulses are continued until the sensed current(solid lines in FIG. 11c) drops slightly below the reference currentcorresponding to the desired one of four states (dashed lines in FIG.11c). To better illustrate this point, assume that each programmingpulse raises V_(tx) linearly by 200 millivolts, and assume further thatthe device was first erased to V_(T1) =-3.2 V. Then the number ofprogramming/sensing pulses required is:

For state "3" (V_(T1) =-3.0 V) No. of pulses=(3.2-3.0)/0.2=1

For state "2" (V_(T1) =-0.5 V) No. of pulses=(3.2-0.5)/0.2=14

For state "1" (V_(T1) =+2.0 V) No. of pulses=(3.2-(-2.0))/0.2=26

and for state "0" (V_(T1) =+4.5 V) No. of pulses=(3.2-(-4.5))/0.2=39.

In actual fact shifts in V_(tx) are not linear in time, as shown in FIG.11b (curve (a)), therefore requiring more pulses than indicated forstates "1" and "0". If 2 microseconds is the programming pulse width and0.1 microseconds is the time required for sensing, then the maximum timerequired to program the device into any of the 4 states is approximately39×2+39×0.1=81.9 microseconds. This is less than the time required by"intelligent programming algorithms" of prior art devices. In fact, withthe new programming algorithm only carefully metered packets ofelectrons are injected during programming. A further benefit of thisapproach is that the sensing during reading is the same sensing as thatduring programming/sensing, and the same reference current sources areused in both programming and reading operations. That means that eachand every memory cell in the array is read relative to the samereference level as used during program/sense. This provides excellenttracking even in very large memory arrays.

Large memory systems typically incorporate error detection andcorrection schemes which can tolerate a small number of hard failuresi.e. bad Flash EEprom cells. For this reason the programming/sensingcycling algorithm can be automatically halted after a certain maximumnumber of programming cycles has been applied even if the cell beingprogrammed has not reached the desired threshold voltage state,indicating a faulty memory cell.

There are several ways to implement the multi-state storage concept inconjunction pith an array of Flash EEprom transistors. An example of onesuch circuit is shown in FIG. 11e. In this circuit an array of memorycells has decoded word lines and decoded bit lines connected to thecontrol gates and drains respectively of rows and columns of cells. Eachbit line is normally precharged to a voltage of between 1.0 V and 2.0 Vduring the time between read, program or erase. For a four statestorage, four sense amplifiers, each with its own distinct currentreference levels IREF, 0, IREF,1, IREF, 2, and IREF, 3 are attached toeach decoded output of the bit line. During read, the current throughthe Flash EEprom transistor is compared simultaneously (i.e., inparallel) with these four reference levels (this operation can also beperformed in four consecutive read cycles using a single sense amplifierwith a different reference applied at each cycle, if the attendantadditional time required for reading is not a concern). The data outputis provided from the four sense amplifiers through four Di buffers (D0,D1, D2 and D3).

During programming, the four data inputs Ii (I0, I1, I2 and I3) arepresented to a comparator circuit which also has presented to it thefour sense amp outputs for the accessed cell. If Di match Ii, then thecell is in the correct state and no programming is required. If howeverall four Di do not match all four Ii, then the comparator outputactivates a programming control circuit. This circuit in turn controlsthe bit line (VPBL) and word line (VPWL) programming pulse generators. Asingle short programming pulse is applied to both the selected word lineand the selected bit line. This is followed by a second read cycle todetermine if a match between Di and Ii has been established. Thissequence is repeated through multiple programming/reading pulses and isstopped only when a match is established (or earlier if no match hasbeen established but after a preset maximum number of pulses has beenreached).

The result of such multistate programming algorithim is that each cellis programmed into any one of the four conduction states in directcorrelation with the reference conduction states I_(REF), i. In fact,the same sense amplifiers used during programming/reading pulsing arealso used during sensing (i.e., during normal reading). This allowsexcellent tracking between the reference levels (dashed lines in FIG.11c) and the programmed conduction levels (solid lines in FIG. 11c),across large memory arrays and also for a very wide range of operatingtemperatures. Furthermore, because only a carefully metered number ofelectrons is introduced onto the floating gate during programming orremoved during erasing, the device experiences the minimum amount ofendurance-related stress possible.

In actual fact, although four reference levels and four sense amplifiersare used to program the cell into one of four distinct conductionstates, only three sense amplifiers and three reference levels arerequired to sense the correct one of four stored states. For example, inFIG. 11c, I_(REF) ("2") can differentiate correctly between conductionstates "3" and "2", I_(REF) ("1") can differentiate correctly betweenconduction states "2" and "1", and I_(REF) ("0") can differentiatecorrectly between conduction states "1" and "0". In a practicalimplementation of the circuit of FIG. 11e the reference levels I_(REF),i (i=0,1,2) may be somewhat shifted by a fixed amount during sensing toplace them closer to the midpoint between the corresponding lower andhigher conduction states of the cell being sensed.

Note that the same principle employed in the circuit of FIG. 11e can beused also with binary storage, or with storage of more than four statesper cell. Of course, circuits other than the one shown in FIG. 11e arealso possible. For example, voltage level sensing rather than conductionlevel sensing can be employed.

c. Improved Charge Retention

In the example above,states "3" and "2" are the result of net positivecharge (holes) on the floating gate while states "1" and "0" are theresult of net negative charge (electrons) on the floating gate. Toproperly sense the correct conduction state during the lifetime of thedevice (which may be specified as 10 years at 125° C.) it is necessaryfor this charge not to leak off the floating gate by more than theequivalent of approximately 200 millivolts shift in V_(T1). Thiscondition is readily met for stored electrons in this as well as allprior art Eprom and Flash EEprom devices. There is no data in theliterature on charge retention for stored holes, because, as has beenpointed out above, none of the prior art devices concern themselves withthe value V_(T1) when it is more negative than V_(T2), i.e., when holesare stored on th floating gate. From device physics considerations aloneit is expected that retention of holes trapped on the floating gateshould be significantly superior to the retention of trapped electrons.This is because trapped holes can only be neutralized by the injectionof electrons onto the floating gate. So long as the conditions for suchinjection do not exist it is almost impossible for the holes to overcomethe potential barrier of approximately 5.0 electron volts at thesilicon--silicon dioxide interface (compared to a 3.1 electron voltspotential barrier for trapped electrons).

Therefore it is possible to improve the retention of this device byassigning more of the conduction states to states which involve trappedholes. For example, in the example above state "1" had V_(T1) =+2.0 V,which involved trapped electrons since V_(T1) for the virgin device wasmade to be. V_(T1) =+1.5 V. If however V_(T1) of the virgin device israised to a higher threshold voltage, say to V_(T1) =+3.0 V (e.g. byincreasing the p-type doping concentration in the channel region 560a inFIG. 5a), then the same state "1" with V_(T1) =+2.0 V will involvetrapped holes, and will therefore better retain this value of V_(T1). Ofcourse it is also possible to set the reference levels so that most orall states will have values of V_(T1) which are lower than the V_(T1) ofthe virgin device.

d. Intelligent Erase for Improved Endurance

The endurance of Flash EEprom devices is their ability to withstand agiven number of program/erase cycles. The physical phenomenon limitingthe endurance of prior art Flash EEprom devices is trapping of electronsin the active dielectric films of the device (see the Wegener articlereferenced above). During programming the dielectric used during hotelectron channel injection traps part of the injected electrons. Duringerasing the tunnel erase dielectric likewise traps some of the tunneledelectrons. For example, in prior art transistor 200 (FIG. 2) dielectric212 traps electrons in region 207 during programming and in region 208during erasing. The trapped electrons oppose the applied electric fieldin subsequent write/erase cycles thereby causing a reduction in thethreshold voltage shift of V_(tx). This can be seen in a gradual closure(FIG. 11d) in the voltage "window" between the "0" and "1" states ofprior art devices. Beyond approximately 1×10⁴ program/erase cycles thewindow closure can become sufficiently severe to cause the sensingcircuitry to malfunction. If cycling is continued the device eventuallyexperiences catastrophic failure due to a ruptured dielectric. Thistypically occurs at between 1×10⁶ and 1×10⁷ cycles, and is known as theintrinsic breakdown of the device. In memory arrays of prior art devicesthe window closure is what limits the practical endurance toapproximately 1×10⁴ cycles. At a given erase voltage, V_(ERASE), thetime required to adequately erase the device can stretch out from 100milliseconds initially (i.e. in a virgin device) to 10 seconds in adevice which has been cycled through 1×10⁴ cycles. In anticipation ofsuch degradation prior art Flash EEprom devices specify a sufficientlylong erase pulse duration to allow proper erase after 1×10⁴ cycles.However this also results in virgin devices being overerased andtherefore being unnecessarily overstressed.

A second problem with prior art devices is that during the erase pulsethe tunnel dielectric may be exposed to an excessively high peak stress.This occurs in a device which has previously been programmed to state"0" (V_(T1) =+4.5 V or higher). This device has a large negative Q (seeequation (2)). When V_(ERASE) is applied the tunnel dielectric ismomentarily exposed to a peak electric field with components fromV_(ERASE) as well as from Q/C_(T) (equations (2) and (3)). This peakfield is eventually reduced when Q is reduced to zero as a consequenceof the tunnel erase. Nevertheless, permanent and cumulative damage isinflicted through this erase procedure, which brings about prematuredevice failure.

To overcome the two problems of overstress and window closure a newerase algorithm is disclosed, which can also be applied equally well toany prior art Flash EEprom device. Without such new erase algorithm itwould be difficult to have a multistate device since, from curve (b) inFIG. 11d, conduction states having V_(T1) more negative than V_(T2) maybe eliminated after 1×10⁴ to 1×10⁵ write/erase cycles.

FIG. 12 outlines the main steps in the sequence of the new erasealgorithm. Assume that a block array of mxn memory cells is to be fullyerased (Flash erase) to state "3" (highest conductivity and lowestV_(T1) state). Certain parameters are established in conjunction withthe erase algorithm. They are listed in FIG. 12: V₁ is the erase voltageof the first erase pulse. V₁ is lower by perhaps 5 volts from the erasevoltage required to erase a virgin device to state "3" in a one seconderase pulse. t is chosen to be approximately 1/10 th of the timerequired to fully erase a virgin device to state "3". Typically, V₁ maybe between 10 and 20 volts while t may be between 10 and 100milliseconds. The algorithm assumes that a certain small number, X, ofbad bits can be tolerated by the system (through for example errordetection and correction schemes implemented at the system level. If noerror detection and correction is implemented then X=0). These would bebits which may have a shorted or leaky tunnel dielectric which preventsthem from being erased even after a very long erase pulse. To avoidexcessive erasing the total number of erase pulses in a complete blockerase cycling can be limited to a preset number, n_(max). ΔV is thevoltage by which each successive erase pulse is incremented. Typically,ΔV is in the range between 0.25 V and 1.0 V. For example, if V₁ =15.0 Vand ΔV=1.0 V, then the seventh erase pulse will be of magnitudeV_(ERASE) =21.0 V and duration t. A cell is considered to be fullyerased when its read conductance is greater than I_("3"). The number Sof complete erase cyclings experienced by each block is an importantinformation at the system level. If S is known for each block then ablock can be replaced automatically with a new redundant block once Sreaches 1×10⁶ (or any other set number) of program/erase cycles. S isset at zero initially, and is incremented by one for each complete blockerase multiple pulse cycle. The value of S at any one time can be storedby using for example twenty bits (2²⁰ equals approximately 1×10⁶) ineach block. That way each block carries its own endurance history.Alternatively the S value can be stored off chip as part of the system.

The sequence for a complete erase cycle of the new algorithm is asfollows (see FIG. 12):

1. Read S. This value can be stored in a register file. (This step canbe omitted if S is not expected to approach the endurance limit duringthe operating lifetime of the device).

1a. Apply a first erase pule with V_(ERASE) =V₁ +n ΔV, n=0, pulseduration=t. This pulse (and the next few successive pulses) isinsufficient to fully erase all memory cells, but it serves to reducethe charge Q on programmed cells at a relatively low erase field stress,i.e., it is equivalent to a "conditioning" pulse.

1b. Read a sparse pattern of cells in the array. A diagonal read patternfor example will read m+n cells (rather than mxn cells for a completeread) and will have at least one cell from each row and one cell fromeach column in the array. The number N of cells not fully erased tostate "3" is counted and compared with X.

1c. If N is greater than x (array not adequately erased) a second erasepulse is applied of magnitude greater by ΔV than the magnitude of thefirst pulse, with the same pulse duration, t. Read diagonal cells, countN.

This cycling of erase pulse/read/increment erase pulse is continueduntil either N≦X or the number n of erase pulses exceed n_(max). Thefirst one of these two conditions to occur leads to a final erase pulse.

2a. The final erase pulse is applied to assure that the array is solidlyand fully erased. The magnitude of V_(ERASE) can be the same as in theprevious pulse or higher by another increment ΔV. The duration can bebetween 1t and 5t.

2b. 100% of the array is read. The number N of cells not fully erased iscounted. If N is less than or equal to X, then the erase pulsing iscompleted at this point.

2c. If N is greater than X, then address locations of the N unerasedbits are generated, possibly for substitution with redundant good bitsat the system level. If N is significantly larger than X (for example,if N represents perhaps 5% of the total number of cells), then a flagmay be raised, to indicate to the user that the array may have reachedits endurance end of life.

2d. Erase pulsing is ended.

3a. S is incremented by one and the new S is stored for futurereference. This step is optional. The new S can be stored either bywriting it into the newly erased block or off chip in a separateregister file.

3b. The erase cycle is ended. The complete cycle is expected to becompleted with between 10 to 20 erase pulses and to last a total ofapproximately one second.

The new algorithm has the following advantages:

(a) No cell in the array experiences the peak electric field stress. Bythe time V_(ERASE) is incremented to a relatively high voltage anycharge Q on the floating gates has already been removed in previouslower voltage erase pulses.

(b) The total erase time is significantly shorter than the fixedV_(ERASE) pulse of the prior art. Virgin devices see the minimum pulseduration necessary to erase. Devices which have undergone more than1×10⁴ cycles require only several more ΔV voltage increments to overcomedielectric trapped charge, which only adds several hundred millisecondsto their total erase time.

(c) The window closure on the erase side (curve (b) in FIG. 11d) isavoided indefinitely (until the device experiences failure by acatastrophic breakdown) because V_(ERASE) is simply incremented untilthe device is erased properly to state "3". Thus, the new erasealgorithm preserves the full memory window.

FIG. 13 shows the four conduction states of the Flash EEprom devices ofthis invention as a function of the number of program/erase cycles.Since all four states are always accomplished by programming or erasingto fixed reference conduction states, there is no window closure for anyof these states at least until 1×10⁶ cycles.

In a Flash EEprom memory chip it is possible to implement efficientlythe new erase algorithm by: providing on chip (or alternatively on aseparate controller chip) a voltage multiplier to provide the necessaryvoltage V1 and voltage increments ΔV to nΔV, timing circuitry to timethe erase and sense pulse duration, counting circuitry to count N andcompare it with the stored value for X, registers to store addresslocations of bad bits, and control and sequencing circuitry, includingthe instruction set to execute the erase sequence outlined above.

VII. Edge Tailored Flash EEprom with New Erase Mechanism

Flash EEprom embodiments 600a, 700a, 800a, and 900a of this inventionuse tunnel erase across a relatively thick dielectric oxide grown on thetextured surface of the polysilicon floating gate. Wegener (see articlereferenced above) has postulated that asperities--small, bump-like,curved surfaces of diameter of approximately 30 nanometers, enhance theelectric field at the injector surface (in this case, the floating gate)by a factor of 4 to 5, thereby allowing efficient tunnel conduction tooccur even across a relatively thick tunnel dielectric film (30 to 70nanometers). Accordingly, there have been in the prior art efforts,through process steps such as high temperature oxidation of thepolysilicon surface, to shape the surface of the polysilicon so as toaccentuate these asperities. Although such steps are reproducible, theyare empirical in nature, somewhat costly to implement, and not wellunderstood.

A new approach is disclosed in this invention which results in a highlyreproducible, enhanced electric field tunnel erase which is moreefficient than the asperities method yet simpler to implement in severalEEprom and Flash EEprom devices. In this approach, the floating gatelayer is deposited in a very thin layer, typically in the range between25 and 200 nanometers. This is much thinner than floating gates of allprior art Eprom, EEprom or Flash EEprom devices, which typically use alayer of polysilicon of thickness at least 200 nanometers, and usuallymore like 350 to 450 nanometers. The prior art polysilicon thickness ischosen to be higher than 200 nanometers primarily because of the lowersheet resistivity and better quality polyoxides provided by the thickerpolysilicon. In certain prior art devices such as the Eitan splitchannel Eprom the floating gate also serves as an implant mask (FIG. 4b)and must therefore be sufficiently thick to prevent penetration of theimplant ions. Likewise, in the split channel Eprom embodiment 500a (FIG.5a) the spacer formation (FIGS. 5b through 5f) cannot be readilyimplemented if floating gate 504a is 100 nanometers or less inthickness. However, Eprom transistor 1400 (FIG. 14c) and Flash EEpromtransistors 600a (FIG. 6a), 700a (FIG. 7a), 800a (FIG. 8a) and 900a(FIG. 9a) as well as the Kupec prior art transistor 200b (FIG. 2b) canall be implemented with a floating gate of thickness 100 nanometers orless to achieve a significant improvement in erase efficiency.

The reason for going to such a thin layer of polysilicon is that theedges of the floating gate in such a thin layer can be tailored throughoxidation to form extremely sharp-tipped edges. The radius of curvatureof these tipped edges can be made extremely small and is dictated by thethickness of the thin polysilicon film as well as the thickness of thetunnel dielectric grown. Therefore, tunnel erase from these sharp tipsno longer depends on surface asperities but instead is dominated by thetip itself.

As an illustration of this modification, consider Flash EEpromtransistor 800a (FIG. 8a) in two different embodiments, a relativelythick floating gate (transistor 800a shown in FIG. 8b and FIG. 16a) andthe same transistor modified to have a very thin floating gate(transistor 800M shown in FIG. 16b). In the cross section view of FIG.16a (corresponding to direction AA of FIG. 8a), floating gate 804a isapproximately 300 nanometers thick. Its vertical edges 862a, 832a areshown having a multitude of small asperities at the surface. Eachasperity acts as an electron injector during tunnel erase (shown by thedirection of the arrows across tunnel dielectric layers 861a, 831a).Injected electrons are collected by erase gates 835, 830 which overlapvertical edges 862a, 832a.

By contrast, the cross section view of modified transistor 800M is shownin FIG. 16b (along the same cross section AA of FIG. 8a) shows atransistor with floating gate 804M of thickness 100 nanometers or less.Dielectric layers 864 and 867 as well as control gate 809 can be thesame as in transistor 800a.

During oxidation of the thin vertical edges of floating gate 804M toform tunnel dielectric layers 861M, 831M, both top and bottom surfacesof the thin floating gate at its exposed edges are oxidized. Thisresults in extremely sharp tips 870l, 870r being formed. These tipsserve as very efficient electron injectors (shown by arrows acrosstunnel dielectrics 861M, 831M). Injected electrons are collected as intransistor 800a by erase gates 835, 830, which overlap thesesharp-tipped edges.

Apart from the very efficient and highly reproducible injectorcharacteristics inherent to the thin floating gate of transistor 800Mthere is an additional benefit in that the capacitance between thefloating gate at its tip and the erase gate is much smaller than thecorresponding capacitance in all other embodiments, including transistor800a. Therefore, from equations (1), (2) and (3) in section VI.a., since

    C.sub.E <<C.sub.T,

Therefore, V_(FG) =Q/C_(T), and

    E.sub.ERASE =(V.sub.ERASE -Q/C.sub.T)/t.

When Q=0 (virgin device), then

    E.sub.ERASE =V.sub.ERASE /.sup.t                           (4)

Equation (4) basically states that when C_(E) is very small relative toC_(T), then essentially 100% of the erase voltage V_(ERASE) iseffectively applied across the tunnel dielectric layer of thickness t.This allows a reduction of the magnitude of V_(ERASE) necessary to erasethe device. Also, a very small C_(E) allows all other devicecapacitances contributing to C_(T) (in FIG. 10) to be made small, whichleads to a highly scalable Flash EEprom device. The thinner floatinggate also helps to improve metalization step coverage and to reduce thepropensity to form polysilicon stringers in the manufacturing process.

Two other points are worth noting. First, the very thin floating gateshould not be overly heavily doped, to avoid penetration of the N+dopant through polysilicon 804M and gate dielectric 864. Since floatinggate 804M is never used as a current conductor, a sheet resistivity ofbetween 100 and 10,000 Ohms per square in quite acceptable.

Secondly, it is necessary to ensure that the sharp tips of the floatinggate are adequately spaced apart or isolated from control gate 809M aswell as substrate 860 or the source or drain diffusions (not shown inFIG. 16b). This is because the sharp tip injection mechanism can be sohighly effective that unintended partial erase to these surfaces maytake place under the voltage conditions prevailing during deviceprogramming (i.e., a "program disturbance" condition). This problem isnot necessarily a severe one because, looking again at equations (1),(2) and (3), capacitance components C_(G), C_(D) and C_(B) are each muchlarger than C_(E) and therefore the electric field between the floatinggate at its edges and any of these three surfaces is much less thanE_(ERASE). Nevertheless, this should be an important consideration inthe actual geometrical layout of any floating gate transistor using avery thin floating gate for edge erase.

Although a thin floating gate layer provides a relatively straightforward approach to achieving after oxidation sharp-tipped edges, otherapproaches are possible to achieve sharp-tipped edges even in arelatively thick floating gate layer. For example, in FIG. 16c arelatively thick layer forming floating gate 804 is etched with areentrant angle of etching. After oxidation, a sharp tip 870 is formedat the top edge, facilitating high field tunneling 861 to the erase gate830 deposited on top of the tunnel erase dielectric 831.

In the device of FIG. 16d the erase gate is deposited before thefloating gate. Erase gate 830 is etched so as to create a reentrantcavity close to its bottom surface. Tunnel erase dielectric 831 is thengrown, followed by deposition and formation of floating gate 804.Floating gate 804 fills the narrow reentrant cavity where a sharp tip870 is formed, which facilitates the high field tunneling 861. Note thatthe device of FIG. 16d has asperities formed at the surface of the erasegate whereas all other devices described in this invention haveasperities formed at the surfaces of their floating gate.

VIII. Flash EEprom Memory Array Implementations

The Flash EEprom cells of this invention can be implemented in densememory arrays in several different array architectures. The firstarchitecture, shown in FIG. 15a, is the one commonly used in theindustry for Eprom arrays. The 3×2 array of FIG. 15a shows two rows andthree columns of Flash EEprom transistors. Transistors T10, T11, T12along the first row share a common control gate (word line) and a commonsource S. Each transistor in the row has its own drain D connected to acolumn bit line which is shared with the drains of all other transistorsin the same column. The floating gates of all transistors are adjacenttheir drains, away from their sources. Erase lines are shown running inthe bit line direction (can also run in the word line direction), witheach erase line coupled (through the erase dielectric) to the floatinggates of the transistors to the left and to the right of the erase line.The voltage conditions for the different modes of operation are shown inTable I (FIG. 17a) for the selected cell as well as for unselected cellssharing either the same-row (word line) or the same column (bit line).During block erase of all the cells in the array, all erase lines arebrought high. However, it is also possible to erase only sectors of thearray by taking V_(ERASE) high for pairs of erase gates only in thesesectors, keeping all other erase lines at 0 V.

A second Flash EEprom memory array architecture which lends itself tobetter packing density than the array of FIG. 15a is known as thevirtual ground array (for a detailed description of this arrayarchitecture, see the Harari patent referenced herein). A topologicalview of such an array of cells was provided in FIGS. 6a, 7a, 8a and 9a.A schematic representation of a 2×2 virtual ground memory arraycorresponding to the array of FIG. 6a is shown in FIG. 15b. In a virtualground array, the source and drain regions are used interchangeably. Forexample, diffusion 502 is used as the drain of transistor 600a and asthe source of transistor 600b. The term "virtual ground" comes from thefact that the ground supply applied to the source is decoded rather thanhard-wired. This decoding allows the source to be used interchangeablyas ground line or drain. The operating conditions in the virtual groundarray are given in Table II (FIG. 17b). They are essentially the same asthat for the standard architecture array, except that all source anddrain columns of unselected cells are left floating during programmingto prevent accidental program disturbance. During reading all columnsare pulled up to a low voltage (about 1.5 V) and the selected cell alonehas its source diffusion pulled down close to ground potential so thatits current can be sensed.

The array can be erased in a block, or in entire rows by decoding theerase voltage to the corresponding erase lines.

While the embodiments of this invention that have been described are thepreferred implementations, those skilled in the art will understand thatvariations thereof may also be possible. In particular, the splitchannel Flash EEprom devices 600a, 700a, 800a and 900a can equally wellbe formed in conjunction with a split channel Eprom composite transistor500a having channel portions L1 and L2 formed in accordance with theone-sided spacer sequence outlined in FIGS. 5b through 5f, or inaccordance with Eprom transistor 1400, or with Eprom transistors formedin accordance with other self-aligning process techniques or, altogetherin non self-aligning methods such as the ones employed in the prior artby Eitan, Samachisa, Masuoka and Harari. Therefore, the invention isentitled to protection within the full scope of the appended claims.

It is claimed:
 1. A method of operating a plurality of blocks ofnonvolatile floating gate memory cells as part of a memory system,comprising:subjecting cells within the plurality of memory blocks to anumber of program/erase cycles, maintaining separate counts of a numberof program/erase cycles experienced by individual ones of said memoryblocks, including storing the counts within the individual blocks towhich the counts pertain, providing at lease one redundant memory blockof memory cells, replacing at least one of said plurality of memoryblocks with said at least one redundant memory block when the countednumber of program/erase cycles of said at least one memory block reachesa set number, and wherein subjecting cells of a given block to a numberof program/erase cycles includes reading the experience count from thegiven block and storing the count in a register file, followed byerasing said given block, and wherein storing the experience countincludes incrementing by one the count read from said given block and,after erasure of said given block, writing the incremented count backinto said given block.
 2. The method according to claim 1, wherein saidset number represents a predetermined limit of a number of program/erasecycles that the memory cells of the individual memory blocks can endurebefore becoming degraded.
 3. The method according to claim 1 whichadditionally comprises programming said at least one redundant memoryblock with user data directed to said at least one replaced memoryblock.
 4. The method according to claim 3, wherein said set numberrepresents a predetermine limit of a number of program/erase cycles thatthe memory cells of the individual memory blocks can endure beforebecoming degraded.